The space between the first floor ceiling and second floor's floor. Is probably at 78°F and 55% RH, or higher. If the return air temp is 74, and the temp difference across the coil is 20 degrees. making the supply duct 54°F. It will still sweat, as the dew point or the conditions I posted is 60.5°F. Even if the temp difference between supply and return ws only 14 degrees, the supply duct would be 60, so just under dew point.